1. Field of the Invention.
This invention relates to a method and system for making reflection holograms and, more specifically, for making reflection holograms from a master having a cylindrical surface.
2. Description of Related Art.
Holography is a form of optical information storage. The general principles are described in a number of references, e.g., "Photography by Laser" by E. N. Leith and J. Upatnieks in Scientific American, 212. No. 6, pages 24-35 (Jun., 1965). A useful discussion of holography is presented in "Holography", by C. C. Guest, in Encyclopedia of Physical Science and Technology. Vol. 6, pages 507-519, R. A. Meyers, Ed., Academic Press, Orlando, Fla., 1987.
In brief, the object to be photographed or imaged is illuminated with coherent light (e.g., from a laser) and a light sensitive recording medium (e.g., a photographic plate) is positioned so as to receive light reflected from the object. This beam of reflected light is known as the object beam. At the same time, a portion of the coherent light is directed to the recording medium, bypassing the object. This beam is known as the reference beam. The interference pattern that results from the interaction of the reference beam and the object beam impinging on the recording medium is recorded in the recording medium. When the processed recording medium is subsequently appropriately illuminated and observed at the appropriate angle, the light from the illuminating source is diffracted by the hologram to reconstruct the wavefront that originally reached the recording medium from the object. Thus, the hologram resembles a window through which a real or virtual image of the object is observed in full three-dimensional form.
Holograms that are formed by allowing the reference and object beams to enter the recording medium from the same side are known as transmission holograms. Interaction of the object and reference beams in the recording medium forms fringes of material with varying refractive indices which are approximately normal to the plane of the recording medium. When the hologram is played back by viewing with transmitted light, these fringes refract the light to produce real and virtual images. Such transmission holograms may be produced by methods which are well known in the art, such as disclosed in U.S. Pat. Nos. 3,506,327, 3,838,903 and 3,894,787 issued to Leith and Upatnieks.
U.S. Pat. No. 4,209,250 discloses of system for making multiple copies from a stationary planar transmission master hologram. A copy medium is transported such that a portion of the copy medium is coupled to the master. Then a beam is directed through the master onto the copy medium exposing the copy medium portion. After exposure, the exposed portion of the copy medium is transported away from the master. This process is repeated to make multiple transmission hologram copies and is referred to as a step and repeat process.
Holograms formed by allowing the reference and object beams to enter the recording medium from opposite sides are known as reflection holograms. Interaction of the object and reference beams in the recording medium forms fringes of material with varying refractive indices which are, approximately, planes parallel to the plane of the recording medium. When the hologram is played back these fringes act as partial mirrors reflecting incident light back to the viewer. Hence, the hologram is viewed in reflection rather than in transmission.
Reflection holograms may be produced by an on axis or an off-axis method. Reflection holograms are produced by the on-axis method when the beam of coherent radiation is projected through the recording medium onto an object therebehind. In this instance, the beam reflected off the object returns and intersects with the projected beam in the recording medium to form fringes substantially parallel to the medium. The on-axis method of producing reflection holograms is described in an article by Yu N. Denisyuk entitled "Photographic Reconstruction of the Optical Properties of an Object in its Own Scattered Radiation Field," published in Soviet Physics--Doklady, 7, pgs. 543-5 (1962). Also see an article by Clark N. Kurtz entitled "Copying Reflection Holograms," published in the Journal of the Optical Society of America, 58, pgs. 856-7 (1968). Reflection holograms are produced by the off-axis method when a reference beam is projected on one side of the recording medium and an object beam is projected on the reverse side of the medium. In this instance, the object beam is formed by illuminating the object with coherent radiation which has not passed through the recording medium. For instance, the original beam of coherent radiation can be split into two portions, one portion being projected on the medium and the other portion being manipulated to project on the object behind the medium. Reflection holograms produced by an off-axis process are disclosed in U.S. Pat. No. 3,532,406.
The substantially horizontal fringes which form reflection holograms are much more difficult to record than the perpendicular fringes which form transmission holograms for two reasons. The first reason is the need for higher resolution, i.e., the need to record more fringes per unit length, and thus a need to record faithfully closer fringes. Horizontal reflection holograms require about three times to six times more fringes per unit length than do transmission holograms. The second reason relevant to photopolymers is the sensitivity of horizontal fringes to shrinkage of the recording medium during exposure Shrinkage of the recording medium during exposure will tend to wash out the fringes and, if severe, will prevent a hologram from being formed. This is in contrast to the transmission hologram case, where shrinkage has little or no effect when the fringes are substantially perpendicular or perpendicular to the plane of the medium, and produces only relatively minor image distortion if the transmission fringes are slanted more than 45.degree. from the plane of the medium.
It is an object of this invention to provide a system capable of making reflection holograms by an on-axis method from a cylindrical master.
It is another object of this invention to provide a system capable of making reflection holograms in a continuous fashion, in addition to a step and repeat manner.